Delayed Coking. Chapter 5

Transcription

1 Delayed Coking Chapter 5

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3 Purpose Process heavy residuum to produce distillates (naphtha & gas oils) that may be catalytically upgraded Hydrotreating, catalytic cracking, and/or hydrocracking Attractive for heavy residuum not suitable for catalytic processes Large concentrations of resins, asphaltenes, & heteroatom compounds (sulfur, nitrogen, oxygen, metals) Metals, sulfur, & other catalyst poisons generally end up in coke Sold for fuel & other purposes Carbon rejection process Coke Drums Coke Fired Heater Steam Fresh Feed Gas Naphtha Light Gas Oil Heavy Gas Oil Fractionator

4 Development of Coking Coking capacity is measured in terms of both coke production in tons per day & residual oil feed rate in barrels per day EIA database as of January 1, 2009 : Unit Crude Units Vacuum Units bbl per stream day 18.2 MMbpd 8.8 MMbpd Relative Capacity 48% Delayed Coking Units 2.5 MMbpd 14% Fluid Coking Units 0.2 MMbpd 1%

5 U.S. Refinery Implementation Company State Site Atmospheric Crude Distillation Capacity (barrels per stream day) Vacuum Distillation Downstream Charge Capacity, Current Year (barrels per stream day) Therm Cracking, Delayed Coking Downstream Charge Capacity, Current Year (barrels per stream day) Therm Cracking, Fluid Coking Downstream Charge Capacity, Current Year (barrels per stream day) Petcoke,Market Production Capacity, Current Year (barrels per steam day except sulfur and hydrogen) ExxonMobil Refining Louisiana BATON ROUGE 524, , , ,525 Access Industries Texas HOUSTON 299, , , ,960 Chevron USA Inc Mississippi PASCAGOULA 360, , , ,500 Valero Energy Corp Texas PORT ARTHUR 320, ,000 99, ,240 PDVSA Louisiana LAKE CHARLES 440, ,000 99, ,000 Deer Park Refining Ltd Partnership Texas DEER PARK 340, ,000 88, ,500 Chevron USA Inc California EL SEGUNDO 294, ,000 83, ,500 ConocoPhillips Texas SWEENY 260, ,100 78, ,800 Valero Energy Corp Louisiana NORCO 186, ,000 71, ,785 BP California LOS ANGELES 265, ,000 67, ,400 Company State Site Atmospheric Crude Distillation Capacity (barrels per stream day) Vacuum Distillation Downstream Charge Capacity, Current Year (barrels per stream day) Therm Cracking, Delayed Coking Downstream Charge Capacity, Current Year (barrels per stream day) Therm Cracking, Fluid Coking Downstream Charge Capacity, Current Year (barrels per stream day) Petcoke,Market Production Capacity, Current Year (barrels per steam day except sulfur and hydrogen) ExxonMobil Refining Texas BAYTOWN 596, ,600 51,400 42,500 22,750 Valero Energy Corp Delaware DELAWARE CITY 190, , ,000 13,620 Royal Dutch/Shell Group California MARTINEZ 158, ,000 25,000 22,500 8,600 Valero Energy Corp California BENICIA 148,000 81, ,500 6,800 ExxonMobil Refining Montana BILLINGS 62,200 28, ,400 4,000 Top 10 Delayed Cokers. All fluidized cokers.

6 Characteristics of Petroleum Products Refining Overview Petroleum Processes & Products, by Freeman Self, Ed Ekholm, & Keith Bowers, AIChE CD-ROM, 2000

7 Development of Coking After World War II railroads shifted from steam to diesel locomotives Demand for heavy fuel oil sharply declined Coking increases distillate production & minimizes heavy fuel oil 1950 to 1970 coking capacity increased five fold More than twice the rate of increase in crude distillation capacity Increase in heavy high sulfur crude combined decrease in heavy fuel oil

8 Coking Chemistry Carbon rejection process Coke has very little hydrogen shifts to the lighter products Metals (hydrotreatingcatalyst poisons) concentrate in coke Cycle of cracking & combining Side chains cracked off of PNA (Polynuclear Aromatic) cores Heteroatomsin side chains end up in light products PNAs combine (condense) to form asphaltenes& coke Metals & heteroatomsin PNA cores end up in coke Conditions High temperatures & low pressures favor cracking More distillate liquids Lower yields of coke & hydrocarbon gas High residence time favor the combining reactions Over conversion will reduce distillates & produce coke and hydrocarbon gases

9 Example Asphaltene Molecule

10 Coking Technologies Provider ConocoPhillips Foster Wheeler / UOP KBR Lummus Technology ExxonMobil Features Delayed Coking with unique features of: furnace design; coke drum structure, design, layout, & scheduling; coke handling Fluidized bed

11 Delayed Coking Predominate coking technology Delayed Coking technology is relatively inexpensive Open art available Companies do license technology emphasizing coke furnaces, special processing modes, & operations

12 Feed for the Delayed Coker Delayed Coker can process a wide variety of feedstocks Can have considerable metals (nickel & vanadium), sulfur, resins, & asphaltenes Most contaminants exit with coke Typical feed is vacuum resid Atmospheric resid occasionally used Typical feed composition 6% sulfur 1,000 ppm(wt) metals ConradsonCarbon Residue (CCR) of wt% or more Feed ultimately depends on type of coke desired

13 Solid Products Coke with large amounts of metals & sulfur may pose a disposal problem Oil sands pile it up Product grades Needle coke Anode grade Fuel grade Product Morphology Needle coke Sponge coke Shot coke

14 Solid Products High quality products Needle coke FCC cycle oils & gas oils Used for electrodes in steel manufacturing Anode grade coke Residswith small ring aromatics, low metals, & low sulfur Used for electrodes in aluminum production Hydroprocessingupstream of delayed cokermay be used to make high quality coke Fuel grade coke About 85-90% carbon, 4% hydrogen, 4-7% sulfur, 1% nitrogen, oxygen, vanadium & nickel Feedstock resid high in polynuclear aromatics & sulfur Value similar to coal

15 Solid Products Morphology Needle coke Very dense & crystaline in structure Sponge coke Is sponge-like in structure Shot coke Cannot avoid based on asphaltenecontent of feed From size of small ball bearings to basketball Operational adjustments required in cutting & handling of coke

16 Shot Coke: Design & Operations, John D. Elliott

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18 Light Products Vapor light ends processed in refinery gas plant Liquids Naphtha fraction May be used as catalytic reformer feed after hydrotreating Small fraction of gasoline pool Light Gas Oil Used in diesel pool after hydrotreating Hydrocracker processes aromatic rings Heavy Gas Oil fed to catalytic cracker or Hydrocracker preferred Flash Zone Gas Oil Increases liquid yield & reduces coke make Composition Reduced aromatics but high olefin content Though heteroatomsare concentrated in coke still high in sulfur

19 Feedstock Selection Amount of coke related to carbon residue of feed Correlates to hydrogen/carbon ratio & indicates coking tendency Three main tests Conradson Carbon (ASTM D 189) Ramsbottom method (ASTM D 524) MicrocarbonResidue Test (ASTM D 4530 )

20 Yields Low yields of liquids relative to hydrocracking Mass conversion of vacuum residsto liquids about 55% about 90% for hydrocracking Coke & liquid yields may be estimated by simple equations (misprint pg. 104 see pg. 117) Coke Yield (wt%) = 1.6 (wt% CCR) Gas (C4-) (wt%) = (wt% CCR) Gasoline (wt%) = (wt% CCR) Gas Oil (wt%) = (wt% Coke) - (wt% Gas) - (wt% Gasoline) Gasoline (vol%) = (wt% Gasoline ) API Gas Oil (vol%) = (wt% Gas Oil) API

21 Product Light Ends & Sulfur Distribution Estimated product distribution Tables 5.8 & 5.9 Typical Gas Composition Component Methane Ethene Ethane Propene Propane Butenes I-Butane N-Butane H2 CO2 Total Mole% Gas Light Naphtha Heavy Naphtha LCGO HCGO Coke Total Typical Distributions Sulfur (%) Nitrogen (%)

22 Use of Yield Equations Liquid Vol% Weight% Mole% Standard Liquid Density Molecular Weight H2 Calc 13.7 * Pure H2S Calc * Pure Pure CO2 Calc 0.2 Pure Pure Gas (C4-) C1 C2= C2 C3= C * %CCR Calc Calc Calc Calc Calc Pure Pure Pure Pure Pure Pure Pure Pure Pure C4=s Calc 2.4 Pure Pure IC4 Calc 1.0 Pure Pure NC4 Calc 2.6 Pure Pure Gasoline (Wt%) * / ( API) * %CCR Calculate Gas Oil (wt%)* / ( API) Calculate Coke 1.6 * %CCR Total 100% 100% Notes: Sulfur in gas as H 2 S. Decrease H 2 amount to account for amount H 2 S. Interrelate the mass of non-sulfur gas using the mol% values above.

23 Example Yield Problem #1 What are the expected products from a delayed cokerwhen running 100,000 sbpdof the Torrance Field crude oil (assay on page 404)? Use the residuum as given in assay.

24 Example Yield Problem #1 What are the expected products from a delayed coker when running 100,000 sbpdof the Torrance Field crude oil (assay on page 404)? Use the residuum as given in assay. Steps Determine volume feed based on vol% yield of vacuum resid. Determine mass feed based on density of vacuum resid. Determine yield percentages based on formulas. Gas Oil Yield is calculated by difference from 100%. Determine amounts based on yield percentages. Determine densities based on volumes & mass produced. Determine the distribution of sulfur based on the typical factors. Scale the sulfur content of the products as wt%. Split up the non-sulfur portion of the cokergas according to the typical composition. Correct for presence of sulfur. Reduce the moles of H 2 replace with appropriate amount of H 2 S.

25 Example Yield Problem #1 CCR Sulfur (wt%) Yield Yield bbl/day lb/day SpGr lb/gal API wt% wt% wt% vol% Crude Charge 100,000 31,899, Vac Resid Feed 41,900 14,730, Coker Gas 1,428, Coker Gasoline 8,770 2,330, Coker Gas Oil 24,669 7,860, Coke 3,111, Coker Total 14,730, Suflur Distribution Coker Gas Composition Sulfur (%) lb/day mol/day Corrected Corrected Corrected Gas ,713 3,983 Component Mol% Mol Wt mol/day mol/day Mol% lb/day Light Naphtha 1.7 7,237 Methane ,167 30, ,962 Heavy Naphtha ,048 Ethene ,697 LCGO ,559 Ethane ,332 9, ,604 HCGO ,439 Propene ,819 1, ,562 Coke ,713 Propane ,813 4, ,220 Total ,710 Butenes ,409 1, ,032 I-Butane ,113 N-Butane ,526 1, ,694 H ,041 4, ,180 CO ,166 H2S , ,742 Sulfur ,983 Total ,674 58, ,428,972 w/o Sulfur ,691

26 Configuration Typical equipment Heater (furnace) & Preheat train Coke drum vessels Fractionator Downstream vapor processing vessels Coke drums run in two batch modes Filling Decoking Both modes of operation concurrently feed to the fractionator

27 Typical Delayed Coking Unit Original Source: Refining Overview Petroleum Processes & Products, by Freeman Self, Ed Ekholm, & Keith Bowers, AIChE CD-ROM, 2000

28 Typical Delayed Coking Unit Fresh Feed & Furnace Fresh feed to bottom of fractionator Total feed (fresh feed + recycle) heated in furnace Furnace Outlet temperature about 925 F cracking starts about 800 F Endothermic reactions Superheat allows cracking reactions to continue in coke drums Delayed Coking Steam injected into furnace Reduce oil partial pressure & increase vaporization Maintains high fluid velocities

29 Typical Delayed Coking Unit Coke Drum Configuration Flow up from bottom Coking reaction are completed in drum Vapors out top of drum to fractionator Even number of coke drums Typically two or four Operate as pairs, one filling while the other decoked

30 Typical Delayed Coking Unit Coke Drum Cyclic Operation Fill Coke Drum Coking reaction in drums & solid coke deposited Gas from top of coke drum to fractionator Full cycle time till coke drum full Decoking Off-line drum decoked Quench step hot coke quenched with steam then water. Gives off steam & volatile hydrocarbons Initial steam purge fed to fractionator. Further purge directed to blowdown system. Coke drilled out with water drills Coke Collection Systems Direct discharge to hopper car Pad loading Pit & crane loading

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32 Coke Drum Schedule Drum Being Filled Drum Being Decoked 1 hour -Steam out 4 hours -Quench 1.5 hours -Dehead 16 hours -Fill drum with coke 4 hours -Drill out coke 1 hour -Rehead 4.5 hours Test, Warmup, & Standby

33 Decoking Each coke drum has a drilling rig that raises & lowers a rotating cutting head Uses high-pressure (4,000 psig) water Steps Drum cooled & displaced with water to remove volatiles Pilot hole is drilled through the coke to bottom head Pilot drill bit replaced with a much larger high-pressure water bit Cut direction predominantly top to bottom Bottom up cutting risks stuck drill if bed collapses The coke falls from coke drum into a collection system

34 Handbook of Petroleum Refining Processes Robert Meyers McGraw-Hill, Inc, 1986 Decoking

35 Decoking Handbook of Petroleum Refining Processes Robert Meyers McGraw-Hill, Inc, 1986

36 Typical Delayed Coking Unit Fractionator Vapors compressed & sent to gas plant Naphtha is condensed from fractionator overhead Gas oils are side stream draws from the fractionator Flash Zone Gas internally recycled to coke drums or recovered as additional liquid product.

37 Coke Products Green Coke Directly produced by a refinery if no further processing done Fuel coke Calcined Coke Green coke heated to finish carbonizing coke & reduce volatile matter to very low levels Anode & needle coke

38 Calcining Green coke heated to finish carbonizing coke & reduce volatile matter to very low levels Calciningdone in rotary kiln or rotary hearth Heated F Calcining does not remove metals Uncalcinedsponge coke has heating values of 14,000 Btu/lb Primarily used for fuel Crushed & drained of free water contains 10% moisture, 10% volatiles, & the rest coke

39 Fluid Bed Coking & Flexicoking Fluid Coking & Flexicokingare expensive processes that have only a small portion of the coking market Continuous fluidized bed technology Coke particles used as the continuous particulate phase with a reactor and burner Exxon Research and Engineering licensor of Flexicoking process Third gasifier vessel converts excess coke to low Btu fuel gas

40 Fluid Bed Coking Coke Recycled to Extinction Figure from

41 Flexicoking Figure from